Many aircraft, particularly in military and unmanned roles, need to operate efficiently at more than one flight regime. For example, certain airframes are expected to perform in relatively high speed cruise modes, as well as slower loitering scenarios. Unfortunately, it is well established that an engine operates most efficiently when the exit velocity closely matches the speed of the airframe. As a result, designing an engine that is suitable for a plurality of roles, yields an engine that only performs moderately well at each of those operating conditions.
One approach to improve performance over a wider range of missions, is to add an additional flow path to the engine that can be turned on and off depending on the required operating requirements. While this method can produce acceptable results, current efforts are geometrically constrained by internal nacelle size. Such a constrained variable cycle engine can only vary its bypass ratio relatively slightly from a low to mid bypass ratio. A less constrained geometry would allow for large variations in bypass ratio allowing for an efficient high bypass turbofan to switch modes to a low bypass turbofan or even turbojet configuration.
If the aircraft needs to be efficient at both high subsonic Mach numbers such as supercruise at Machs greater than 1.6 as well as low subsonic Mach numbers for efficient loiter such as Machs between 0.7 and 0.9, a different approach is necessary to design an engine that can operate efficiently through a wider range of operational speeds. Current designs do not allow for the a user to select between a high bypass turbofan mode in order to operate efficiently at slow Mach numbers efficiently, and the option to turn off the high bypass portion of the engine in order to convert the engine into a low bypass turbofan (or even turbojet) to operate efficiently at higher Mach regimes.
Therefore, there a need exists for an improved variable bypass turbofan engine capable of selectively switching between high bypass and low bypass turbofan configurations.